Astronomers have used pulsating stars to trace the crooked shape of our galaxy’s disk.

warped galaxy disk
This artist's illustrations shows our galaxy's warped disk, traced out by young variable stars called Cepheids (green points).
J. Skowron / OGLE / Astronomical Observatory, University of Warsaw

A careful survey of more than 2,400 Cepheid variable stars has revealed the Milky Way’s warped disk in new detail. Dorota Skowron (University of Warsaw, Poland) and colleagues report the result in the August 2nd Science.

Cepheids are giants and supergiants that breathe in and out at a rate proportional to their intrinsic brightness. This period-luminosity relation makes them superb distance markers, as Henrietta Swan Leavitt discovered in the early 20th century.

Skowron used the Optical Gravitational Lensing Experiment (OGLE), as well as data from five other surveys and catalogs, to map the Cepheids’ 3D locations, which lie primarily within a few tens of thousands of light-years of the Sun. Their project confirms there’s a severe warp in our galaxy’s disk, reminiscent of pizza dough bent in its mid-toss flight. The warp has also shown up in maps using neutral hydrogen gas, stars, dust, and stellar motions, as well as a recent infrared study that used roughly half as many Cepheids as Skowron’s team did.

When they plotted the variable stars’ locations looking down at our galaxy’s disk, the astronomers noticed that the Cepheids clump, gathering in several concentrations that trace out a sloppy spiral pattern. Curious, the team took the three most prominent clumps and calculated the ages of the stars in them. They found that the stars in each group had a similar age to one another — approximately 64, 113, and 175 million years. The youngest clump’s stars tightly clustered together, whereas the oldest clump's stars were the most spread out.

The team thinks that these Cepheid populations were born in three bursts of star formation. As time passed, stars that formed together would have naturally gone their separate ways, explaining why the oldest stars are the most spread out. Computer simulations confirm that three starbirth episodes would have stretched into the pattern the team’s map reveals in the Milky Way.

Since all three rounds of star formation happened on the same side of the galaxy, Skowron speculates that an encounter with a dwarf galaxy might have triggered them.

Astronomers have reconstructed the Milky Way's 3D structure based on the precisely measured distances of more than 2,400 Cepheids (colored dots). The Sun icon marks our position.
J. Skowron / OGLE / Astronomical Observatory, University of Warsaw

Reference: D. M. Skowron et al. “A Three-Dimensional map of the Milky Way Using Classical Cepheid Variable Stars.” Science. August 2, 2019.


Image of Tomasz Kokowski

Tomasz Kokowski

August 2, 2019 at 12:39 pm

Distorted Science?

OGLE team article has been sent to Scence at the end of May in 2018.
A month later under agreement with Science this article has been released
in Arxiv (
Next month - it's only at the end of July 2018 another team of mostly chinese astronomers submitted to Nature their article ( about the distorted shape of Galaxy dish. This article was based on two times smaller and lower quality cepheid sample than OGLE sample was.
The OGLE article has been presented on IAU Congress in August 2018.
The OGLE article review has been completed in 2018.
The Chinese team article appeared in February issue of Nature.
The OGLE article appeared only in August issue of Science.
Any conclusions?

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August 2, 2019 at 5:43 pm

Unless I am mistaken, this is not a 3D representation but a 4D.
Though it is not stated in the article, I am assuming that the locations used in this representation are the apparent locations of the objects as viewed from the earth at a very recent point in time. So time zero is at the solar icon, and the time is linear backwards from it, with the distance (e.g. 1000 light years = 1000 years ago). I don't imagine anyone has tried to create a 3D image at a single point in time by using the currently observed motions of these stars and extrapolating to the current time. But it might be interesting and informative.

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